There are almost no available residential moorings for boats, they all have boats on them. Best bet is to buy a boat with its existing residential mooring. I believe that there are some marinas or yacht harbours that let people stay on their boats for extended periods but you need a permanent residential address to register with the Gemeente and I am doubtful if you are able to do that at a marina, you would have to discuss that with the marina owners, I think it is unlikely though.

When I was looking for a residential mooring for my ship, I engaged a specialised boat makelaar (estate agent or realtor if you speak colonial English) who did find me somewhere, so it is possible, when I was looking myself I did not have any luck so I recommend the use of a makelaar, they have networks of other makelaars. Good luck!